1. Field of the Invention
The present invention relates in general to the field of laser light amplification.
2. Related Art
Chirped Pulse Amplification (CPA) is very useful for producing ultrashort-duration high-intensity pulses for use in high peak power ultrashort pulse laser systems. CPA increases the energy of an ultrashort laser pulse while avoiding optical amplifier damage and excessive nonlinear distortion. In this technique, the duration of the pulse is increased by first dispersing the ultrashort laser pulse temporally as a function of wavelength (a process called “chirping”) to produce a chirped pulse, then amplifying the chirped pulse, and then recompressing the chirped pulse to significantly shorten its duration. Lengthening the pulse in time reduces the peak power of the pulse and, thus, allows energy to be added to the pulse without incurring excessive nonlinearities or reaching a damage threshold of the pulse amplifier and optical components. The amount of pulse amplification that can be achieved is typically proportional to the amount of pulse stretching and compression. Typically, the greater the amount of stretching and compression, the greater the possible pulse amplification.
A CPA system typically comprises an optical stretcher, an optical amplifier, and an optical compressor. The optical stretcher and optical compressor are ideally configured to have equal but opposite dispersive properties to perfectly compensate for one another to minimize the pulse width of an amplified optical pulse. Any material through which an optical pulse propagates may add dispersion to the optical pulse. For example, another optical component such as the optical amplifier may add dispersion to the optical pulse that is not compensated even by a perfectly matched optical stretcher and compressor pair.
The optical stretcher may comprise a bulk diffraction grating, an optical fiber, a fiber grating, or other dispersive optical elements. Optical fiber-based dispersive optical elements are generally not used in the optical compressor because the peak power of an optical pulse within the optical compressor is generally larger than an optical fiber's nonlinear threshold. Therefore, bulk diffraction gratings are generally used in optical compressors due to the ability of bulk diffraction gratings to handle larger optical power levels than optical fibers. In a CPA system with a bulk grating-based stretcher and a bulk grating-based compressor, any desired pulse shaping or dispersion compensation would typically be done in the stretcher to provide finer phase control and better resolution.
There is a motivation to use a same type of dispersive element (albeit with opposite dispersion properties) in both the optical stretcher and the optical compressor in order for the optical compressor to achieve maximum dispersion compensation of the optical stretcher. In addition, there is a motivation to use compact dispersive elements such as optical fiber instead of bulk diffraction gratings wherever possible because the compact dispersive elements enable systems to be more compact and to be optically aligned with greater ease. Hybrid chirped pulse amplifier systems balance benefits and drawbacks of different dispersive elements by using different technologies in the optical stretcher and the optical compressor. For example, a hybrid CPA system may use a fiber-based optical stretcher and a bulk diffraction grating-based optical compressor.
Mismatched dispersive element technologies in an optical stretcher and an optical compressor of a hybrid CPA system lead to mismatched dispersion properties. For example, an optical stretcher comprising an optical fiber may have different dispersion properties than an optical compressor comprising a bulk optical grating, because an optical fiber material has different optical properties than a bulk optical grating. Consequently, the optical compressor cannot adequately compensate for the dispersion of the optical stretcher in higher orders, such as fourth order, fifth order, sixth order, and above. These higher order dispersion terms cause temporal spreading in an output optical pulse, and consequently prevent the output optical pulse from reaching the picosecond (ps) or femtosecond (fs) time durations that are desired. In a typical hybrid CPA system, the stretcher and compressor are designed to be approximately matched to lower orders, such as group velocity dispersion and/or third order dispersion.
Mismatched dispersion properties of the optical stretcher and optical compressor in a hybrid CPA system, combined with additional dispersion in the optical amplifier, have motivated development of techniques for compensation of the dispersion mismatch to achieve either the shortest possible output optical pulse width or the most accurate arbitrary optical waveform possible. However, to date, no dispersion compensation techniques capable of compensating for dispersion matches above the fifth order in a hybrid CPA system have been reported. Even at fifth order, the reported dispersion compensation technique in U.S. Pat. No. 5,847,863 entitled “Hybrid Short-Pulse Amplifiers with Phase-Mismatch Compensated Pulse Stretchers and Compressors” and issued Dec. 8, 1998 required that the spatially chirped beam be nonlinearly chirped, which is difficult because the nonlinear spatial chirp is much smaller than the linear spatial chirp. Furthermore, in U.S. Pat. No. 5,847,863, Galvanauskas et al. teach that at high energies, the Treacy configuration is preferable for pulse compression because the absence of any additional material between gratings make the optical compressor less susceptible to optical damage and nonlinear effects.